could someone help me to paraphrase this :
However, in the last decade of the twentieth century and into the twenty-first century the world faced huge movements of people, both within and between countries, that were both proportionately and absolutely the largest migrations in history. Europe and the United States, which have historically long been migrant target areas, faced new waves of incomes but the scale of these was dwarfed by the massive internal movement of migrants inside China, as people moved from the countryside to the cities to provide workers for the country.
Western countries must face the vital question of whether replacement migration provides a solution to population decline and population ageing. Replacement migration means the international migration needed both to offset declines in the size of population in the population of working age and to offset the overall ageing of the population. According to UN statistics, major countries that have large populations and below-replacement fertility levels are France, Germany, Italy, Japan, Republic of Korea, the Russian Federation, and the United Kingdom. The process of population ageing – the transformation of the age structure to relatively greater proportions in the older age groups – is brought about by declining fertility and increased longevity. In most cases, those populations that are simultaneously ageing and declining will experience severe reductions in the ratio of persons of working age (15-64 years) to older persons (65 years and above).